This invention relates to the data signal transmission technique for differential ranging systems using Global Positioning System (hereinafter referred to as GPS) satellites, said differential ranging systems having applications including a navigation system for such mobile terminals mounted on as automobiles, vessels, and aircraft and for the exploration of underground resources and maritime resources.
Drawings are used to show the configurations of conventional differential ranging systems. FIG. 5 shows the configuration of a commonly used differential ranging system and FIG. 6 the configuration of a differential ranging system available from Racal Survey Corporation.
In the Figures, A is a fixed reference station, B is a mobile station, Sa to Sc are GPS satellite orbit information received from GPS satellites 4a to 4c, 1 is a GPS receiver installed at fixed reference station A and mobile station B, 2 is a data processing interface, 3 is a radio that links by radio channel fixed reference station A and mobile station B, 4a to 4c are GPS satellites, and 5 is a radio channel.
4.alpha. is INMARSAT satellite made available by International Maritime Satellite Organization (hereinafter referred to as INMARSAT satellite), C is the mobile bodies like a vessel, 6 and 7 are links of INMARSAT satellite 4.alpha., D is a GPS receiver, E is a transmitter/receiver for using INMARSAT satellite 4.alpha., and F is an earth station for transmitting data to INMARSAT satellite 4.alpha..
Conventional differential ranging systems have had problems such as those described below.
First, a commonly used differential ranging system such as the one shown in FIG. 5 achieves the differential ranging function by deriving the correcting factor at fixed reference station A by using GPS receiver 1 and data processing interface 2, and sending the error information from radio 3 installed at fixed reference station A to mobile station B via radio channel 5.
In this case, however, it is necessary to employ at mobile station B radio 3, apart from GPS receiver 1, and a receiving antenna, as well as data processing interface 2 for processing the data between radio 3 that receives the correcting factor and existing GPS receiver 1. These requirements have made the differential ranging system as a whole quite large, which has created the problems of cost and installation space when used as a navigation system for automobiles, vessels, etc.
In addition, since the conventional differential ranging system required a separate frequency on radio channel 5 over the link for transmitting the correcting factor, it posed a problem also in terms of effective use of radio frequencies.
In the differential ranging system supplied by Racal Survey Corporation as shown in FIG. 6, fixed reference station A uses INMARSAT satellite 4.alpha. to transmit differential data signals over circuits 6 and 7 to user C on a vessel, etc.
In this case, as user C needs to install, apart from GPS receiver D, transmitter/receiver E on his vessel to use INMARSAT satellite 4.alpha. links 6 and 7, the system makes it impossible, due to its cost and space requirement, to install the necessary equipment on a small vessel or automobile, and has forced the user to seek an alternative means.
The invention, to solve said problems with conventional differential ranging systems, provides a differential data signal transmission technique inclusive of the correcting factor that requires no transmitter/receiver other than the GPS receiver.